(57) A semiconductor device comprises a DSP (Digital Signal Processor), a CPU for controlling the DSP and an interface circuit. The interface circuit comprises an input circuit, a gain-adjusting circuit and an output circuit. The input circuit inputs a digital signal and supplies the signal to the DSP synchronously with a first clock signal. The gain-adjusting circuit is capable of adjusting the gain of the digital signal supplied to the input circuit. The output circuit adds a digital signal received from the DSP to the digital signal with the gain thereof adjusted and outputs a signal obtained as a result of the addition synchronously with the first clock signal. A signal path extended from the input circuit through the gain-adjusting circuit to the output circuit forms hardware. A digital signal to be transmitted propagates through the hardware to be subjected to the so-called side-tone processing. As a result, it is possible to provide a semiconductor device capable of generating a side tone without increasing the magnitude of a load borne by a CPU and without requiring an external circuit component.